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has holes of different sizes so that 

 light from the mirror may be modified. 



Optical Axis. This is an imaginary line which 

 passes from the center of the eye-piece through the 

 body, objective, stage and sub-stage to the mirror. 

 Whatever lies in it is said to be centered. 



Object. That which is examined and placed 

 upon a slide. 



Slide. This is a thin plate of glass, generally 

 3 inches long by 1 inch wide. 



Cover Glass. This is an extremely thin piece 

 of glass, round or square, which is placed upon the 

 object, either for flattening or preserving it, or both. 



Classification of Microscopes. Up to within 

 recent years microscopes have been divided into two 

 classes : the Jackson and Ross models. While the 

 latter was for many years very popular, particularly 

 with the English makers, it has been almost entirely 

 superseded by the Jackson form, and with good 

 reason. In the former the means of adjusting were 

 provided, as near as consistent with the construction, 

 to the body or tubes, whereas in the Ross they are 

 placed at the back or more distant point in the 

 instrument, thus increasing by means of the con- 

 necting arm the faults which might exist in the 

 adjustment. 



A certain form of instrument which at the present 

 is very popular and called the Continental pattern, 

 from the fact that it was made originally by the 



